The invention is directed to improvements in the construction of Downs-type electrolytic cells. In particular, it is directed to changes in the riser/cooler by which the operation of the cell is enhanced considerably.
A considerable proportion of the elemental alkali metals which are manufactured for commerce is produced by the electrolysis of molten halogen salts of the metals, especially eutectic mixtures of such salts with other salts which are substantially inert. For example, sodium metal can be produced by electrolysis of a molten binary eutectic mixture comprising calcium chloride and sodium chloride or a ternary eutectic mixture such as sodium chloride, calcium chloride and barium chloride. On the other hand, lithium metal is produced by electrolysis of a molten binary eutectic mixture comprising potassium chloride and lithium chloride.
The type of electrolytic cell most widely used for the above-described operations is the Downs cell, which is described in U.S. Pat. No. 1,501,756 to J. C. Downs. The Downs-type electrolytic cell basically is comprised of a refractory-lined steel shell for holding the molten salt electrolyte, a submerged cylindrical graphite anode surrounded by a cylindrical steel cathode and a perforated steel diaphragm positioned in the annular space between the electrodes to separate the anode and cathode products. To collect product halogen gas from the anode, the cell is provided with collector means such as an inverted cone which fits over the anode below the surface of the molten bath. Halogen gas (usually chlorine) passes upwardly through the cone and, via appropriate manifold components, from the cell. Similarly, the cathode is also provided with collector means such as an inverted inclined trough which fits over the cathode below the surface of the molten bath. Molten alkali metal rises from the cathode toward the surface of the molten bath, is collected along the inclined surface of the trough and is passed to a vertical riser/cooler in which the molten metal is partially cooled before it is passed to a product receiver. Commercial Downs cells frequently contain a plurality of electrode assemblies, in which case the anode product collectors are manifolded together to provide a single gas outlet from the cell. Usually, a common cathode product collector is provided which surrounds all of the gas collector cones. That is, the means for collecting the halogen gas is positioned within the perimeter of the molten alkali metal collector. Detailed illustrations and descriptions of such cells can be found in the following U.S. Patents:
______________________________________ R. E. Hulse U.S. 2,130,801 Sept. 20, 1938 J. S. Honea 2,770,364 Nov. 13, 1956 A. L. Fentress 2,770,592 Nov. 13, 1956 G. O. Hoyes 2,944,950 July 12, 1970 C. T. Gallinger 3,037,927 June 5, 1962 G. T. Motock 3,085,967 April 16, 1963 S. E. Eckert and 3,118,827 Jan. 21, 1964 F. J. Ross J. M. Wood 3,248,311 April 26, 1966 L. L. Harris 3,463,721 Aug. 26, 1969 ______________________________________
In the past, the riser/cooler on Downs-type electrolytic cells has been comprised of a vertical tube rising from the upper end of inverted trough collector through which the molten metal rises by difference in gravity into the bottom of a molten product receiver or, alternatively, up to and over an open weir located at some predetermined higher level in the side of the riser/cooler so that the molten product spills over into the receiver. Though such arrangements have been satisfactory during normal cell operation, they have nevertheless presented serious problems of safety. More particularly, the prior art arrangement of open weirs or pipes between the riser and the receiver results in contamination of the inert gas-purged vapor space of the receiver whenever hydrogen or chlorine are produced inadvertently on the cathode side of the cell. This, of course, increases the amount of inert gas which must be used for operation of the cell and reduces the margin of safety in operation of the cell.